1. Field of the Invention
The present invention relates to a transimpedance-type amplifier circuit capable of interchanging a resistance in response to the amplitude of an output signal from the amplifier, and, more specifically to a transimpedance-type amplifier circuit for amplifying an output signal from a photodetector.
2. Description of the Related Art
A transimpedance-type amplifier circuit for amplifying an output signal from a photodetector has been proposed in Japanese Patent Application Laid-Open No. Hei9-008563 entitled "optical reception preamplifier" as shown in FIG. 7.
Referring to FIG. 7, the preamplifier is constituted of a signal amplifier 50 and a signal control circuit 51. For a purpose of eliminating amplitude differences in output data in relation to input data such as burst type input data, first, the signal reception circuit 50 receives an photocurrent signal or input current signal 13 from a photodiode 15. The photocurrent signal 13 is then supplied into an inverting amplifier 1 and into a buffer circuit 2 as an amplified signal. An output signal 14 from the buffer circuit 2 is compared with a reference voltage signal 12 inputted to a controller 8. The signal control circuit 51 also has a flip-flop circuit 11 actuated by a reset signal 41. According to a result of comparing the output signal 14 from the buffer circuit 2 with the reference voltage signal 12, a resistance value of an amplifier made up of the inverting amplifier 1 and buffer circuit 2 is altered by operation of transistors 5 and 6.
In consideration of the transimpedance-type amplifier circuit described above, the output signal 14 from the buffer circuit 2 is saturated when the value of the optical current signal 13 becomes high during a time required for interchanging the resistance. In the case of saturating the output signal 14, the electric charge increases with a parasitic capacitance 16 of the photodiode 15 since there is no current path through which the photocurrent signal 13 can flow from the photodiode 15. Thus, the output signal 14 from the transimpedance-type amplifier circuit 15 remains saturated if the electric charge of the parasitic capacitance 16 is not discharged, so that the photocurrent signal 13 may not be converted correctly as a photoelectric conversion used for optical communications and the like.